Electron emitting cathodes are used in a variety of low pressure plasma devices, where low pressure is defined as extending downward from a maximum of about 10 millitorr (1.3 Pascal). They are used in gridded ion sources, as described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745. They are also used in gridless ion sources, as described in U.S. Pat. No. 4,862,032--Kaufman, et al. Ion thrusters also use electron emitting cathodes, as described in U.S. No. Pat. 5,359,254--Arkhipov, et al. Ion thrusters are generally similar to industrial ion sources, except that they are used for space propulsion instead of industrial applications. Note that the ion sources described generate broad ion beams that require the presence of charge neutralizing electrons within the ion beam in order to operate.
A beam of energetic ions together with the charge neutralizing electrons constitutes a plasma. Ion sources may therefore also be called plasma sources.
Electron emitting cathodes are also used in other devices such as magnetrons, as described in U.S. Pat. No. 4,588,490--Cuomo, et al.
The specific form of the electron emitting cathode can vary. The simplest is a hot filament of a refractory metal, such as tungsten or tantalum, as described in the aforementioned article by Kaufman, et al., in the AIAA Journal. A hot filament has an important advantage in that the electron emission is directly controllable by adjusting the electrical power used to heat the hot filament.
A hot filament is subject to space-charge limitations, which means that it must be immersed in a plasma to achieve close electrical coupling with that plasma--that is, without an excessive voltage between the cathode and the plasma. For example, a hot filament cannot be used as a neutralizer (to current neutralize an ion beam) in a gridded ion source without being immersed in the beam of energetic ions that it is neutralizing. It also has the shortcoming of having a short lifetime when it is exposed to bombardment by energetic ions. The lifetime problem becomes more severe when reactive gases such as oxygen are present.
Another form of electron emitting cathode is the hollow cathode, as described in U.S. Pat. No. 3,515,932--King, U.S. Pat. No. 3,523,210--Ernstene, et al., and U.S. Pat. No. 5,359,254--Arkhipov, et al. In a hollow cathode, there is an ionizable gas flowing into a cavity and out an aperture. The emission in a hollow cathode is also thermionic, perhaps enhanced with high-field emission due to the dense internal plasma. In operation, a plasma extends from the inside of the cavity, through the aperture, to the surrounding plasma. The heating for the emitting surface inside the cavity comes from ion bombardment. If the voltage to extract electrons is increased, this increased voltage appears as an increased voltage between the emitting surface and the plasma inside the cavity, resulting in turn in an increase in bombardment energy for the ions striking the emitting surface, an increase in temperature of that surface, and therefore an increase in emission. Experimentally, a wide range of electron emission is possible for only small changes in coupling voltage.
Compared to a hot filament, a hollow cathode couples easily to the surroundings, without the space-charge limitations of the former. This ease of coupling results from the "plasma bridge" that extends through the aperture to the surrounding device and/or discharge plasma and provides the ions to charge-neutralize the electrons that are emitted. The plasma bridge permits the hollow cathode, when used as a neutralizer, to be located outside of the energetic ion beam, thereby avoiding erosion by the energetic ions in that beam.
A hollow cathode usually also has a longer lifetime than a hot filament, although reactive gases can also reduce this lifetime. Depending on details of construction and operation of a hollow cathode, the flow of ionizable gas through the aperture may tend to exclude an external reactive gas from the sensitive emitting surface inside the cavity.
There is also a tendency for the bulk of the emission to come from the emitting surface closest to the aperture, resulting in preferential erosion or consumption of the emitting material in that location. This tendency results from the plasma density, ionic bombardment, and heating being greatest near the aperture, which in turn results in the greater emission at that location.
The wide range of electron emission that is possible from a hollow cathode with little variation in coupling voltage can be an advantage in some applications, but a disadvantage in others where the ability to electrically control or limit the emission is important. Complicated electronic circuitry external to the hollow cathode is required to control or limit the emission.
Yet another electron emitting cathode is what is often called the plasma bridge type. It should be noted that "plasma bridge" is used both in the name of an electron emitting cathode and in the description of operation of some electron emitting cathodes. The possible confusion is unfortunate, but is inherent in the language used in the scientific literature. The emission in the plasma bridge cathode is also thermionic, but depends on an external source of electrical power for heating. The thermionic emission can be directly from a hot filament within the cavity, as described in an article by Reader, et al., in the Journal of Vacuum Science and Technology, Vol. 15 (1978), beginning on page 1093. Or the thermionic emission can be from an emitter within the cavity that is heated indirectly by an electrically energized heating element also within the cavity, as described in U.S. Pat. No. 4,297,615--Goebel, et al.
The plasma bridge type of electron emitting cathode also has a plasma extending from inside the cavity, through the aperture, to the surrounding plasma, similar to the plasma bridge in the hollow cathode. The plasma bridge type also has a close electrical coupling with the surrounding device and/or plasma similar to the hollow cathode.
The plasma bridge cathode thus shares some advantages and disadvantages with both the hot filament and hollow cathode. It shares the close electrical coupling and moderate resistance to reactive gases with the hollow cathode. It also shares both the advantage of control of emission and the shortcomings of a hot filament with the hot filament cathode.
The foregoing types of electron emitting cathodes are the most common types used in low pressure plasma devices. The adverse environment of ion bombardment in these devices prevents the use of electron emitting cathodes with delicate emission enhancing surfaces, such as oxides, that are directly exposed to, and unprotected from, surrounding plasmas. Thoriated tungsten has similar shortcomings. The thoria is distributed through the tungsten, but conditioning thoriated tungsten for use results in a surface condition that is rapidly destroyed by ion bombardment.